This invention relates to a rechargeable electrolytic battery fabricated from a unitary flexible laminate cell sheet of polymeric electrode and separator elements, and electrically-conductive foil current collector elements. In particular, the invention relates to a method of shaping and sizing such a battery by folding the laminate cell sheet in a manner which enables the battery terminals to be located on the collector elements at positions which minimizes the distance of current flow through the collectors and thus reduces the impedance encountered within the battery.
Versatile rechargeable battery cells, such as lithium-ion intercalation cells, are currently prepared from electrode elements comprising flexible sheets of polymeric composition in which are dispersed finely-divided particulate materials capable of reversibly intercalating lithium ions during battery charge/discharge cycles. Such materials may include, as positive electrode components, lithium metal oxide intercalation compounds, e.g., LiCoO2, LiNiO2, and LiMn2O4, and, as negative electrode components, carbon materials, such as petroleum cokes and graphites.
Included in the cell structures are flexible electrode-interposed separator layer elements comprising polymers of essentially the same type as employed in the electrode elements, thus facilitating thermal lamination of the element layers to ultimately form the battery composite. Metallic foil electrical current collector elements are also incorporated into the battery cell structure in a laminating operation which essentially embeds these current collectors into the surface of the respective electrode layer elements.
A laminated battery cell representative of present structures is depicted in FIG. 1 of this specification, and the general process of battery cell fabrication is described in greater detail in U.S. Pat. No. 5,460,904 and its related patent specifications, incorporated herein by reference, which discuss typical compositions and procedures for formulating and laminating composite lithium-ion cells.
The simple single cell battery, a representative section of which is shown in FIG. 1, is typically about 0.4 mm thick and may be fabricated as individual sheets of any desired area or, if a continuous process is employed, as a single sheet several meters long which, due to the flexibility of the laminate, may be stored in roll form prior to later processing and conversion to final battery size.
As has been noted in the above-mentioned references, among others, the general performance of the polymeric laminated battery depends upon the composition and structure of the electrode elements. That is to say, the operating voltage range of a battery cell is determined by the electromotive potential difference between the active materials comprising the respective electrodes, while the electrical energy storage capacity depends upon the amount of such materials contained in the cell. The ultimate battery voltage may thus be varied by electrically connecting a number of cells in series and its capacity may be increased by arranging a number of single cells in parallel, increasing the thickness or number of electrode layers in a cell, or increasing the area, i.e., the dimensional size, of a single cell. Of the alternatives affecting battery capacity, the latter is the most feasible, since less extraneous materials, such as connecting conductors, and fewer processing steps are required, and the flexibility and higher functional speed of a thinner laminate are maintained, as well.
Practical disadvantages arise in the latter practice, however, not the least among which is the fact that a single cell of useful capacity would be of unwieldy dimension if allowed to remain as a planar sheet. For this reason it has been normal practice to utilize a cell in the form of an elongate strip-like sheet of practical width and either roll the sheet tightly about its lesser transverse axis or fold the sheet repeatedly at parallel such axes in zig-zag or accordion fashion to obtain a multi-layer cell of desired rectangular dimensions.
Unfortunately, other disadvantages arise in these procedures. Notably, in the rolling scheme, there is required an extraneous insulating sheet to prevent short-circuiting between the otherwise contacting opposite-polarity faces of the cell laminate. On the other hand, while not hampered by this problem due to the fact that the faces of each folded segment of a laminate cell contact a like-polarity face, the folded cell shares with the rolled cell the additional disadvantage that in order to be readily accessed for attachment of conductor wire or the like, a terminal must be located on the face of a current collector layer of the cell laminate at a point near the outermost end of the cell strip. As a result of such a location, electrical current being drawn from the terminal must traverse the longest available path through the entire length of the current collector layer and will therefore be most affected by the impedance inherent in the collector material. The degrading effect of this situation is most prominent in the presence of high current densities or in collector materials of lesser conductivity, such as the aluminum typically associated with the positive battery cell electrodes, or where the thickness of the collector layer element is greatly reduced in the interest of weight limitation.
The purpose of the present invention is therefore to alleviate these problems associated with the previous restriction on terminal location in folded laminate battery cells by means of a unique folding method which provides outside face access to current collector layer segments situated at the more centrally located regions of a battery cell strip. In addition, the problems are addressed at their source in the aspect of the method of cell laminate folding which greatly reduces the inordinate length of the initial single cell in favor of a cell with more equilateral dimensions.
In with accordance one embodiment of the present invention, an elongate strip-shaped laminate battery cell, preferably the simple single cell which presents the thinnest and most flexible cross-section, is folded in such a manner about parallel, spaced transverse axes defining substantially equal longitudinal segments of the cell that contacting electrode collector surfaces are always of the same polarity.
Such a general folding pattern would appear, at the outset, to be similar to the zig-zag folds of the prior art which is depicted in FIG. 2. However, whereas the prior folding scheme comprised an uninterrupted fold series of repeated alternating direction, i.e., forward or reverse fold, the present method intersperses within the fold sequence a number of repeated same-direction folds. For example, in the prior procedure of FIG. 2, the forward-reverse folding sequence, which may be more graphically considered in terms of clockwise (CW) and counter-clockwise (CCW), an odd number of segments e.g, five, so-selected to preserve the outward-lying disposition of one each of the positive and negative electrode collectors, are formed from the fold sequence of CW-CCW-CW-CCW. An embodiment of the invention, on the other hand, as shown in FIG. 3, is formed of the fold sequence of CW-CCW-CCW-CW.
As a result of the irregular fold sequence of the invention, at least one of the collector surfaces, preferably that of the collector material of higher electrical resistance, located at or closely adjacent to the centrally-disposed segment of the cell strip is exposed at the outer surface of the folded structure and may bear an accessible battery terminal. Such a terminal location serves to minimize the collected current travel distance and the resultant impedance within the collector layer.
In a further embodiment of the invention, the initial single cell laminate sheet is cut to a substantially equilateral shape which at once minimizes the collected current travel distance to any terminal location on a cell of given surface area. From this initial shape the cell is folded in alternating directions along parallel axes to obtain the odd number of segments, preferably three, which ensures outwardly-disposed collector surfaces of opposite polarity. According to the invention, the cell thus-folded is again folded in like manner into three segments along parallel axes lying orthogonal to the original fold axes. In this manner the initial cell is reduced to a battery of one-ninth the exposed area having a collector surface of each polarity to which the respective terminals may be affixed while providing the minimum impedance resulting from collected current travel distance.